Investigation: Harriet Harman sports the garment at PMQs as she sits next to Ed Miliband

A pro-feminism t-shirt worn by Ed Miliband, Nick Clegg and Harriet Harman are to be withdrawn from sale following allegations they were made in a sweatshop.

The t-shirts, adorned with the message “This is what a feminist looks like”, are made by women workers being paid just 62p an hour, the Mail on Sunday reported.

They were allegedly made in a factory on the island of Mauritius where the women machinists sleep 16 to a room.

Whistles, which sell the T-shirts for £45 each, have launched an investigation following the reports.

The t-shirts hit the headlines last week when Mr Miliband and Mr Clegg posed in them for photographs for Elle magazine as part of a campaign by the Fawcett Society - which receives all the profits - to promote women's rights.

After David Cameron refused repeated requests to join them, Ms Harman, the Labour deputy leader, then wore one at Prime Minister's Questions in an attempt to embarrass him.

The Mail on Sunday, which toured one of six factories on the island owned by Compagnie Mauricienne de Textile, (CMT) which produces the garments, reported the workers were earning just 6,000 rupees a month - equivalent to £120.

The paper said the figure was just a quarter of the country's average monthly wage, and around half of what a waiter earns.

Fayzal Ally Beegun, president of the International Textile, Garment and Leather Workers Union, told the paper: "The workers in this factory are treated very poorly and the fact that politicians in England are making a statement using these sweatshop t-shirts is appalling."

Dr Eva Neitzert, deputy chief executive at the Fawcett Society, said they had been assured by Whistles that the t-shirts were produced to "ethical standards".

She said they had originally been assured the garments would be produced ethically in the UK, and when they received samples in early October they noted they had in fact been made in Mauritius.

They were assured by Whistles that the factory was "a fully audited, socially and ethical compliant factory" and decided to continue with the collaboration.

"We have been very disappointed to hear the allegations that conditions in the Mauritius factory may not adhere to the ethical standards that we, as the Fawcett Society, would require of any product that bears our name," she said.

"At this stage, we require evidence to back up the claims being made by a journalist at the Mail on Sunday. However, as a charity that campaigns on issues of women's economic equality, we take these allegations extremely seriously and will do our utmost to investigate them.

"If any concrete and verifiable evidence of mistreatment of the garment producers emerges, we will require Whistles to withdraw the range with immediate effect and donate part of the profits to an ethical trading campaigning body.

"Whilst we wish to apologise to all those concerned who may have experienced adverse conditions, we remain confident that we took every practicable and reasonable step to ensure that the range would be ethically produced and await a fuller understanding of the circumstances under which the garments were produced."

A spokesman for Whistles told the Mail: "We place a high priority on environmental, social and ethical issues. The allegations regarding the production of t-shirts in the CMT factory in Mauritius are extremely serious and we are investigating them as a matter of urgency.

A spokesman for the Deputy Prime Minister said: "Nick Clegg had no idea where these t-shirts were being made and can only assume that the Fawcett Society were unaware of the origins, or they would not have asked him to wear it.

"He remains entirely supportive of efforts to ensure all women are treated as equals in this country and the world over."

A Labour Party spokesman said: "This was a campaign run by Elle and the Fawcett Society to promote feminism and we were happy to support it. Anything else is a matter for Elle magazine and the Fawcett Society."